null check to avoid occasional npe

js/reveal.js

@@ -3090,6 +3090,8 @@
 
 		var iframe = event.target;
 
+		if( iframe && iframe.contentWindow ) {
+
 			// YouTube postMessage API
 			if( /youtube\.com\/embed\//.test( iframe.getAttribute( 'src' ) ) && iframe.hasAttribute( 'data-autoplay' ) ) {
 				iframe.contentWindow.postMessage( '{"event":"command","func":"playVideo","args":""}', '*' );
@@ -3105,6 +3107,8 @@
 
 		}
 
+	}
+
 	/**
 	 * Stop playback of any embedded content inside of
 	 * the targeted slide.